Diesel combustion may generate emissions, including particulate matter (PM). The particulate matter may include diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, PM can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometer range of 100 nanometers. Various technologies have been developed for identifying and filtering out exhaust PMs before the exhaust is released to the atmosphere.
As an example, PM or soot sensors may be used in vehicles having internal combustion engines. A PM sensor may be located upstream and/or downstream of a diesel particulate filter (DPF), and may be used to sense PM loading on the filter and diagnose operation of the DPF. Resistive PM sensors typically include planar interdigitated electrodes that sense a PM or soot load based on a correlation between a measured change in electrical conductivity between a pair of electrodes and the amount of PM deposited between the measuring electrodes. Specifically, the interdigitated electrodes are formed on a common substrate, and the measured conductivity across the electrodes provides a measure of soot accumulation. As such, the electrostatic fields in the planar electrodes are stronger near the surface of the electrodes, and occur tangential to the electrode surface. In addition, the electrostatic fields decay rapidly at distances away from the electrode surface. As a result, soot particles flowing close to the surface of the electrode experiences sufficient electrostatic forces to be trapped onto the electrode surface, while other soot particles may escape. This can lead to poor soot capture and distribution. Further, due to the planar geometry of the sensing electrodes, soot may be accumulated just along one surface (the surface including the electrodes, for example). Consequently, most of the soot in the exhaust stream may go undetected, leading to reduced sensor sensitivity.
One example PM sensor design is shown by Heimann et al. in WO 2006027287. Therein, the interdigitated electrodes are distributed radially around a cylindrical surface thereby increasing the surface area for soot adsorption, and further increasing sensor sensitivity.
However, the inventors herein have recognized potential issues with such an approach. The interdigitated electrodes formed on the cylindrical surface described by Heinmann et al. may continue to have reduced soot capture due to the poor electrostatic attraction experienced by the soot particles located away from the sensor surface. In particular, even with the cylindrical surface, the electrostatic field generated between the electrodes remains tangential to the surface of the sensor. Consequently, the soot particles may experience stronger electrostatic attraction when they are closer to the sensor surface, while other soot particles may continue to escape undetected by the sensor. Further, the sensor output may be affected by the presence of contaminants and/or water droplets impinging on the sensor surface.
The inventors have identified an approach to at least partly address these issues while improving the sensitivity of the PM sensor. In one example approach, PM sensor reliability may be increased by a particulate matter sensor comprising an outer, non-perforated tube with a plurality of negative electrodes along an inner surface, a central, perforated element with a plurality of positive electrodes along an outer surface of the central element, the central element positioned within the outer tube, and an inner tube appended to the central element, each of the outer tube, the central element, and the inner tube having a common axis. In this way, by forming the positive and negative electrodes on different surfaces separated by a gap, the electrostatic fields may be generated normal to each of the surfaces, and may be more uniform in the gap between the electrode surfaces. Thus, soot distribution and accumulation across the electrodes of the sensor may be more uniform.
As one example, an exhaust PM sensor may be configured with sensor electrodes and may be positioned downstream of a particulate filter in an exhaust pipe. The PM sensor may include an outer cylindrical protection tube and an inner guiding tube. The outer tube may protect the sensor electrodes while the guiding tube may guide the exhaust gas towards the sensor electrodes positioned within the outer tube. The guiding tube may include a smaller inner cylindrical non-perforated tube coupled to a larger hollow central element. The inner tube may further trap larger particulates and water droplets in the exhaust stream at an inlet of the inner tube, thereby stopping them from impinging on the sensor electrodes. The central element may include a plurality of perforations through which exhaust gas may be released from the inner tube into a gap formed between the central element and the outer tube. In one example, the central element may be an extension of the inner tube, extending towards a center of the outer tube, and may be positioned centrally within the outer tube.
Sensor electrodes may include a plurality of positive electrodes and a plurality of negative electrodes formed on different surfaces of the central element and the outer tube and may be separated from each other by the gap. Specifically, the negative electrodes may be formed on an inner surface of the outer tube, while the positive electrodes may be formed on an outer surface of the central element. The exhaust entering the PM sensor via the inlet may be directed towards the perforated central element. The exhaust may then flow through the perforations of the central element into the gap separating the sensor electrodes. Soot particles in the exhaust may experience a uniform electric field in the gap due to the separation of the plurality of positive electrodes from the plurality of negative electrodes. As a result, the particles may be evenly deposited in the gap between formed between the sensor electrodes. Sensor regeneration may be initiated once a sufficient amount of soot has been accumulated in the gap.
In this way, by positioning positive and negative electrodes of a PM sensor on surfaces of distinct members of the sensor assembly, and by separating the positive and negative electrodes by the gap, an electrostatic field may be generated across the gap that is normal to each of the electrode surfaces. The technical effect of separating the electrodes and generating electrostatic fields that are normal in the gap between the electrodes is that the electrostatic field generated in the gap may be rendered more uniform. As a result, soot in the exhaust stream may be deposited more uniformly across the electrode surfaces. In addition, by generating electrostatic fields across the gap, electrostatic field does not decay in the gap between the sensor electrodes and hence all soot particles in the gap experience substantially similar electrostatic field. Overall, these characteristics of the sensor assembly may improve the accuracy and reliability of the PM sensor. As such, this increases the accuracy of particulate matter load estimation on a particulate matter filter. In addition, PM sensor sensitivity fluctuations due to impingement of large particulates on the sensing electrodes may be reduced. By enabling more accurate diagnosis of an exhaust DPF, exhaust emissions compliance may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.